Wafer processing apparatus

ABSTRACT

A laser processing apparatus including a condenser having a function of spherical aberration. Since the condenser has a function of spherical aberration, the focal point of a laser beam to be focused by the condenser and applied to a wafer can be continuously changed in position along the thickness of the wafer. Accordingly, a uniform shield tunnel composed of a fine hole and an amorphous region surrounding the fine hole can be formed so as to extend from the front side of the wafer to the back side thereof, by one shot of the laser beam.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of and claims priority to U.S. patentapplication Ser. No. 15/584,228 filed on May 2, 2017, the entirecontents of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a laser processing apparatus forperforming laser processing to a wafer formed of silicon, sapphire,silicon carbide, or gallium nitride, for example.

Description of the Related Art

A plurality of devices such as integrated circuits (ICs), large scaleintegrations (LSIs), light emitting diodes (LEDs), surface acoustic wave(SAW) devices, and power devices are formed on the front side of a waferso as to be separated from each other by a plurality of crossingdivision lines. The wafer thus having the devices on the front side islaser-processed along each division line by a laser processing apparatusto form a division start point along each division line. The wafer isthen divided along each division line where the division start point isformed, thereby obtaining individual device chips. The device chips thusobtained are used in electrical equipment such as mobile phones,personal computers, and illumination equipment (see Japanese PatentLaid-open No. 1998-305420, for example). The laser processing apparatusis composed generally of a chuck table for holding a workpiece, laserbeam applying means having condenser for applying a laser beam to theworkpiece held on the chuck table, and feeding means for relativelyfeeding the chuck table and the laser beam applying means, whereby thelaser beam is applied along each division line formed on a wafer as theworkpiece with high accuracy, thereby forming a division start pointalong each division line where the wafer is to be divided intoindividual device chips.

Further, in general, a laser processing apparatus for forming such adivision start point is classified into a type such that a laser beamhaving an absorption wavelength to the workpiece is applied to performablation as described in Japanese Patent Laid-open No. 1998-305420 and atype such that a laser beam having a transmission wavelength to theworkpiece is applied in the condition where the focal point of the laserbeam is set inside the workpiece, thereby forming a modified layer (seeJapanese Patent No. 3408805, for example). In either type, however, thelaser beam must be applied plural times (in plural passes) along eachdivision line, so as to completely cut the wafer, causing a reduction inproductivity.

To cope with this problem, the present applicant has developed andproposed a technique of suitably setting the numerical aperture of afocusing lens for focusing a laser beam having a transmission wavelengthto a wafer as a workpiece and applying the laser beam to the wafer,according to the refractive index of the material forming the wafer,thereby forming a plurality of shield tunnels along each division lineas a division start point, wherein each shield tunnel extends from thefront side of the wafer to the back side thereof, and each shield tunnelis composed of a fine hole and an amorphous region surrounding the finehole (see Japanese Patent Laid-open No. 2014-221483, for example). Inthe technique disclosed in the above publication, the focusing lensincluded in condenser constituting laser beam applying means is providedby an aspherical lens with the shape of its convex or concave surfaceadjusted or by the combination of plural lenses, whereby the laser beamis focused at one point on an optical axis passing through a work pointin the wafer.

SUMMARY OF THE INVENTION

According to the conventional shield tunnel formed by the laserprocessing apparatus disclosed in Japanese Patent Laid-open No.2014-221483, a weak division start point can be formed without the needfor applying a laser beam plural times (by plural shots) at the samework point. However, it is not easy to form a plurality of desireduniform shield tunnels along each division line according to a change inmaterial or thickness of the wafer. In particular, the strength(weakness) of each shield tunnel to be formed along each division lineis largely dependent upon the power of the laser beam to be applied tothe wafer. Accordingly, the power of the laser beam must be suitablyadjusted so that each shield tunnel to be formed along each divisionline has a predetermined strength. However, in the case of setting thefocal point of the laser beam at one point inside a thinned wafer toform a plurality of shield tunnels along each division line, it isdifficult to uniformly form the plural shield tunnels over the entirelength of each division line. As a result, in dividing the wafer intoindividual device chips by applying an external force to the wafer afterforming the shield tunnels along each division line, there is a problemsuch that a relatively large external force (e.g., 40 N) is required.

It is therefore an object of the present invention to provide a laserprocessing apparatus which can form a plurality of shield tunnels insidea wafer along each division line where the wafer can be divided withoutthe need for a large external force.

In accordance with an aspect of the present invention, there is provideda laser processing apparatus comprising: a chuck table for holding awafer; laser beam applying means for applying a laser beam to said waferheld on said chuck table; and a feeding mechanism for relatively feedingsaid chuck table and said laser beam applying means; said laser beamapplying means including a laser oscillator for oscillating said laserbeam, and a condenser having a focusing lens for focusing said laserbeam oscillated by said laser oscillator and applying said laser beamfocused to said wafer held on said chuck table; said condenser having afunction of spherical aberration such that a focal point to be formed bysaid laser beam passing through a radially inner portion of saidcondenser is continuously changed in position toward said chuck tablefrom a focal point to be formed by said laser beam passing through aradially outer portion of said condenser; said laser beam being appliedto said wafer in the condition where the focal point of said laser beamis set inside said wafer so as to be continuously changed in positionalong the thickness of said wafer, thereby forming a shield tunnelinside said wafer, said shield tunnel being composed of a fine hole andan amorphous region surrounding said fine hole.

Preferably, the function of spherical aberration is realized by thefocusing lens having spherical aberration. Alternatively, the functionof spherical aberration is realized by the focusing lens and a focalpoint correcting plate located between the focusing lens and the chucktable for correcting the position of the focal point of the laser beamto be focused by the focusing lens. Preferably, the range of change inposition from the focal point to be formed by the laser beam passingthrough the radially outer portion of the condenser to the focal pointto be formed by the laser beam passing through the radially innerportion of the condenser is set to 50 to 2000 μm.

According to the laser processing apparatus of the present invention,the condenser has a function of spherical aberration which means afunction of continuously changing the position of the focal point of alaser beam to be focused by the condenser and applied to a wafer, alongthe thickness of the wafer. Accordingly, a uniform shield tunnelcomposed of a fine hole and an amorphous region surrounding the finehole can be formed so as to extend from the front side of the wafer tothe back side thereof, by one shot of the laser beam. As a result, theenergy of the laser beam applied to the wafer can be effectively used toform a good shield tunnel. That is, a uniform shield tunnel can beformed without the need for particularly increasing the power of thelaser beam to be applied, so that the strength of the shield tunnelfunctioning as a division start point can be reduced. Accordingly, thewafer can be divided into individual device chips by applying anexternal force weaker than that in the prior art.

The above and other objects, features and advantages of the presentinvention and the manner of realizing them will become more apparent,and the invention itself will best be understood from a study of thefollowing description and appended claims with reference to the attacheddrawings showing a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general perspective view of a laser processing apparatusaccording to a preferred embodiment of the present invention;

FIG. 2A is a schematic diagram showing the configuration of laser beamapplying means included in the laser processing apparatus shown in FIG.1;

FIG. 2B is a view similar to FIG. 2A, showing a modification;

FIG. 3 is a schematic diagram for illustrating a focal position informing a shield tunnel by using the laser beam applying means shown inFIG. 2A;

FIG. 4A is a perspective view for illustrating a shield tunnel formingstep;

FIG. 4B is a schematic perspective view of a shield tunnel; and

FIG. 4C is a sectional view of a wafer in which a plurality of shieldtunnels have been formed.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A preferred embodiment of the laser processing apparatus according tothe present invention will now be described with reference to theattached drawings. FIG. 1 is a general perspective view of a laserprocessing apparatus 40 according to this preferred embodiment. Thelaser processing apparatus 40 includes a base 41, a holding mechanism 42for holding a workpiece such as a wafer, moving means 43 for moving theholding mechanism 42, laser beam applying means 44 for applying a laserbeam to the workpiece held by the holding mechanism 42, imaging means45, and control means (not shown) configured by a computer. The controlmeans functions to control each means mentioned above.

The holding mechanism 42 includes a rectangular X movable plate 51mounted on the base 41 so as to be movable in an X direction, arectangular Y movable plate 53 mounted on the X movable plate 51 so asto be movable in a Y direction, a cylindrical support 50 fixed to theupper surface of the Y movable plate 53, and a rectangular cover plate52 fixed to the upper end of the support 50. The cover plate 52 isformed with an elongated hole 52 a extending in the Y direction. Acircular chuck table 54 as holding means for holding the workpiece isrotatably mounted on the upper end of the support 50 so as to extendupward through the elongated hole 52 a of the cover plate 52. A circularvacuum chuck 56 is provided on the upper surface of the chuck table 54.The vacuum chuck 56 has a substantially horizontal holding surface. Thevacuum chuck 56 is formed of a porous material. The vacuum chuck 56 isconnected through a suction passage formed in the support 50 to suctionmeans (not shown). A plurality of clamps 58 are provided on the outercircumference of the chuck table 54 so as to be spaced in thecircumferential direction thereof. The X direction is defined as thedirection shown by an arrow X in FIG. 1, and the Y direction is definedas the direction shown by an arrow Y in FIG. 1, which is perpendicularto the X direction in an XY plane. The XY plane defined by the Xdirection and the Y direction is a substantially horizontal plane.

The moving means 43 includes X moving means 60, Y moving means 65, androtating means (not shown). The X moving means 60 includes a ball screw60 b extending in the X direction on the base 41 and a motor 60 aconnected to one end of the ball screw 60 b. The ball screw 60 b has anut portion (not shown), which is fixed to the lower surface of the Xmovable plate 51. The X moving means 60 is operated in such a mannerthat the rotational motion of the motor 60 a is converted into a linearmotion by the ball screw 60 b and this linear motion is transmitted tothe X movable plate 51, so that the X movable plate 51 is moved in the Xdirection along a pair of guide rails 43 a provided on the base 41.Similarly, the Y moving means 65 includes a ball screw 65 b extending inthe Y direction on the X movable plate 51 and a motor 65 a connected toone end of the ball screw 65 b. The ball screw 65 b has a nut portion(not shown), which is fixed to the lower surface of the Y movable plate53. The Y moving means 65 is operated in such a manner that therotational motion of the motor 65 a is converted into a linear motion bythe ball screw 65 b and this linear motion is transmitted to the Ymovable plate 53, so that the Y movable plate 53 is moved in the Ydirection along a pair of guide rails 51 a provided on the X movableplate 51. The rotating means is built in the support 50 to rotate thechuck table 54, or the vacuum chuck 56 with respect to the support 50.

An L-shaped casing 46 is provided on the base 41 at its rear endportion. The L-shaped casing 46 is composed of a vertical portionextending upward from the upper surface of the base 41 and a horizontalportion extending from the upper end of the vertical portion in asubstantially horizontal direction. FIG. 2A is a block diagramschematically showing the configuration of the laser beam applying means44. As shown in FIG. 2A, the laser beam applying means 44 includes alaser oscillator 44 b for oscillating a laser beam LB, an attenuator 44c for adjusting the power of the laser beam LB oscillated from the laseroscillator 44 b, a reflector plate 44 f for reflecting the laser beam LBoutput from the attenuator 44 c, and a condenser 44 a having a focusinglens 44 d for focusing the laser beam LB reflected by the reflectorplate 44 f and applying the same to a wafer 10 as the workpiece held onthe chuck table 54. The laser beam LB to be oscillated by the laseroscillator 44 b has a wavelength (e.g., 1030 nm) for forming a pluralityof shield tunnels inside the wafer 10 along each division line, whereineach shield tunnel is composed of a fine hole and an amorphous regionsurrounding the fine hole.

The control means (not shown) included in the laser processing apparatus40 is configured by a computer, which includes a central processing unit(CPU) for performing operational processing in accordance with a controlprogram, a read only memory (ROM) preliminarily storing the controlprogram, a random access memory (RAM) for temporarily storing detectionvalues, operational results, etc., an input interface, and an outputinterface. The input interface functions to input an image signal fromthe imaging means 45 and detection signals from X position detectingmeans and Y position detecting means (both not shown) included in theholding mechanism 42, for example. The X position detecting meansfunctions to detect the X position of the chuck table 54 in the Xdirection, and the Y position detecting means functions to detect the Yposition of the chuck table 54 in the Y direction. The output interfacefunctions to output operation signals to the laser oscillator 44 b, theX moving means 60, and the Y moving means 65, for example.

As shown in FIG. 1, the imaging means 45 is provided on the lowersurface of the front end portion of the casing 46. The imaging means 45is positioned above the guide rails 43 a. Accordingly, by moving thechuck table 54 along the guide rails 43 a, the imaging means 45 canimage the workpiece held on the chuck table 54. In this preferredembodiment, the imaging means 45 is constituted of an imaging device(charge coupled device (CCD)) (not shown) for imaging the workpiece byusing visible light. As a modification, the imaging means 45 may furtherinclude an infrared light source for applying infrared light to theworkpiece and an infrared imaging device (infrared CCD) for outputtingan electrical signal corresponding to the infrared light applied.

The condenser 44 a will now be described in more detail with referenceto FIGS. 2A and 3. The condenser 44 a essentially includes the focusinglens 44 d. According to the technical idea of the present invention, thefocusing lens 44 d has a function of spherical aberration such that afocal point P1 to be formed on the optical axis of the focusing lens 44d by a laser beam LB1 passing through a radially inner portion of thefocusing lens 44 d is displaced (continuously changed in position)toward the chuck table 54 from a focal point Pn to be formed on theoptical axis of the focusing lens 44 d by a laser beam LBn passingthrough a radially outer portion of the focusing lens 44 d. Thisfunction of spherical aberration may be realized by a spherical lensgenerally known in the art. However, the present invention is notlimited to this configuration, but an aspherical lens or the combinationof plural spherical lenses and aspherical lenses may be adopted torealize the function of spherical aberration. That is, any configurationmay be adopted, provided that the focal points P1 to Pn can bepositioned inside the wafer 10 so as to be displaced in the directionwhere the laser beam LB enters the wafer 10, i.e., along the thicknessof the wafer 10. Further, the range of displacement of the focal pointsP1 to Pn is preferably set to 50 to 2000 μm. This range of displacementis not necessarily required to be larger than the thickness of the wafer10 as the workpiece. For example, in the case that the thickness of thewafer 10 is 300 μm, the focal points P1 to Pn are preferably displacedover the range from the front side of the wafer 10 to the back sidethereof, i.e., over the wafer thickness of 300 μm, whereby each shieldtunnel can be formed more satisfactorily.

The operation of the laser processing apparatus 40 will now be describedwith reference to the drawings. Referring to FIG. 4A, there is shown aperspective view of the wafer 10. In this preferred embodiment, thewafer 10 is formed from a lithium tantalate (LiTaO₃) substrate. Thefront side of the wafer 10 is formed with a plurality of crossingdivision lines 12 to define a plurality of separate regions where aplurality of SAW devices 14 are formed. The back side of the wafer 10 isattached to an adhesive tape T supported at its peripheral portion to anannular frame F. Thus, the wafer 10 is supported through the adhesivetape T to the annular frame F in the condition where the front side ofthe wafer 10 is exposed.

First, the wafer 10 supported through the adhesive tape T to the annularframe F is placed on the vacuum chuck 56 of the chuck table 54 in thecondition where the adhesive tape T is oriented downward. Further, theannular frame F is held by the clamps 58. Thereafter, the suction means(not shown) connected to the vacuum chuck 56 is operated to apply avacuum to the vacuum chuck 56, thereby holding the wafer 10 through theadhesive tape T on the vacuum chuck 56 under suction.

After holding the wafer 10 on the vacuum chuck 56 under suction, the Xmoving means 60 and the Y moving means 65 are operated to move the chucktable 54 and thereby position the wafer 10 directly below the imagingmeans 45. In the condition where the wafer 10 held on the chuck table 54is positioned directly below the imaging means 45, an alignment step isperformed by the imaging means 45 and the control means (not shown) todetect a target area of the wafer 10 to be laser-processed. Morespecifically, the imaging means 45 and the control means (not shown)perform image processing such as pattern matching for making thealignment between the division lines 12 extending in a first directionon the wafer 10 and the condenser 44 a of the laser beam applying means44 for applying a laser beam along these division lines 12. Similarly,the alignment step is performed also for the other division lines 12extending in a second direction perpendicular to the first direction.

After performing the alignment step for all of the division lines 12,the chuck table 54 is moved to a laser beam applying area where thecondenser 44 a is located. Further, one end of a predetermined one ofthe division lines 12 extending in the first direction is positioneddirectly below the condenser 44 a. Thereafter, focal position adjustingmeans (not shown) included in the laser processing apparatus 40 isoperated to move the condenser 44 a along its optical axis and therebyset the focal point P of the laser beam LB at a predetermined positioninside the lithium tantalate substrate of the wafer 10. Morespecifically, as shown in FIGS. 3 and 4C, the laser beam LB1 is a laserbeam passing through a radially innermost portion of the focusing lens44 d. The focal point P1 to be formed by the laser beam LB1 is setinside the wafer 10 at a position near the chuck table 54, i.e., nearthe back side of the wafer 10. On the other hand, the laser beam LBn isa laser beam passing through a radially outermost portion of thefocusing lens 44 d. The focal point Pn to be formed by the laser beamLBn is set inside the wafer 10 at a position near the front side of thewafer 10 where the devices 14 are formed. Accordingly, the focal point Pto be formed by the laser beam LB passing through the focusing lens 44 dof the condenser 44 a is displaced over the range from the back side ofthe wafer 10 to the front side thereof. In FIGS. 3 and 4C, the laserbeam LB is so shown as to be decomposed into the laser beams LB1 to LBn,and the focal point P of the laser beam LB is so shown as to bedecomposed into the focal points P1 to Pn for convenience ofillustration. However, in actual, the laser beam LB is not seen so as tobe decomposed as shown.

After setting the focal point P as described above, the laser beamapplying means 44 is operated to oscillate the laser beam LB (pulsedlaser beam) from the laser oscillator 44 b. The laser beam LB oscillatedfrom the laser oscillator 44 b is input into the attenuator 44 c, inwhich the power of the laser beam LB is adjusted to a predeterminedvalue. Thereafter, the laser beam LB is focused by the condenser 44 aand applied to one end of the predetermined division line 12 on thewafer 10. When the application of the laser beam LB is started, the Xmoving means 60 is operated to move the chuck table 54 in the directionshown by an arrow X in FIG. 4A, thereby scanning the laser beam LB alongthe predetermined division line 12. Accordingly, as shown in FIGS. 4Band 4C, a plurality of shield tunnels 100 are continuously formed alongthe predetermined division line 12, wherein each shield tunnel 100 iscomposed of a fine hole 102 extending vertically (axially over theentire length of each shield tunnel 100) and a cylindrical amorphousregion 104 for shielding the fine hole 102 so as to surround the same.This laser processing is similarly performed along all of the otherdivision lines 12 formed on the front side of the wafer 10 by operatingthe laser beam applying means 44, the chuck table 54, the X moving means60, and the Y moving means 65, thereby forming a plurality of similarshield tunnels 100 along each division line 12 (shield tunnel formingstep). After performing the shield tunnel forming step, the wafer 10 istransferred to any apparatus for performing a dividing step of dividingthe wafer 10 into individual device chips corresponding to therespective devices 14 by applying an external force to the wafer 10.This dividing step is not essential in the present invention. As theapparatus for performing the dividing step, any dividing means known inthe art may be used (see Japanese Patent Laid-open No. 2014-221483, FIG.8 and its related description, for example). Accordingly, the detaileddescription of the dividing step and the dividing means will be omittedherein.

For example, the shield tunnel forming step is performed under thefollowing processing conditions.

Wavelength: 1030 nm

Average power: 3 W

Repetition frequency: 50 kHz

Pulse width: 10 ps

Spot diameter: 10 μm

Numerical aperture of the focusing lens/Refractive index of the wafer:0.05 to 0.20

Work feed speed in the X direction: 500 mm/second

Size of each shield tunnel: fine hole 1 μm in diameter, amorphous region10 μm in diameter

As described above, the condenser 44 a in the laser processing apparatus40 has a function of spherical aberration such that the focal point P1to be formed by the laser beam LB1 passing through the radially innerportion of the condenser 44 a is displaced toward the chuck table 54from the focal point Pn to be formed by the laser beam LBn passingthrough the radially outer portion of the condenser 44 a, wherein thelaser beam LB composed of the laser beams LB1 to LBn is applied to thewafer 10 in the condition where the focal point P of the laser beam LBis set inside the wafer 10, thereby forming the shield tunnel 100composed of the fine hole 102 and the amorphous region 104 surroundingthe fine hole 102. Accordingly, the energy of the laser beam LB can beeffectively used for the formation of the shield tunnel 100, so that aplurality of good shield tunnels 100 can be continuously formed alongeach division line 12 over the entire length thereof. Further, eachshield tunnel 100 can be formed over the entire thickness of the wafer10, so that each division line 12 can be sufficiently reduced instrength over the entire length. Accordingly, in the dividing step, thewafer 10 can be easily divided into the individual device chips byapplying a relatively small external force.

In this preferred embodiment, the focusing lens 44 d of the condenser 44a has a function of spherical aberration such that the focal point P1 tobe formed by the laser beam LB1 passing through the radially innerportion of the focusing lens 44 d is displaced toward the chuck table 54from the focal point Pn to be formed by the laser beam LBn passingthrough the radially outer portion of the focusing lens 44 d. Thisconfiguration is merely illustrative, and the present invention is notlimited to this configuration. FIG. 2B shows a modification of the aboveconfiguration. As shown in FIG. 2B, condenser 44 a′ having a focusinglens 44 e and a focal point correcting plate 44 g may be used in placeof the condenser 44 a. The focal point correcting plate 44 g is locatedbetween the focusing lens 44 e and the chuck table 54. The focusing lens44 e has a lens surface for forming a focal point at one position insidethe wafer 10. The focal point correcting plate 44 g is located betweenthe focusing lens 44 e and the wafer 10 held on the chuck table 54, andit has a function of correcting the focal point of the laser beam havingpassed through the focusing lens 44 e in such a manner that the focalpoint to be formed by the laser beam passing through a radially innerportion of the focal point correcting plate 44 g is displaced toward thechuck table 54 from the focal point to be formed by the laser beampassing through a radially outer portion of the focal point correctingplate 44 g.

The above-mentioned function of the focal point correcting plate 44 gmay be realized by using glass as the material of the focal pointcorrecting plate 44 g and forming a spherical convex surface as the beamentrance surface of the focal point correcting plate 44 g where thelaser beam enters. By locating the focal point correcting plate 44 g atthe above-mentioned position, the traveling direction of the laser beamLB having passed through the focusing lens 44 e is corrected by passingthe laser beam through the focal point correcting plate 44 g, that is,the traveling direction of the laser beam is changed between at theradially inner portion of the focal point correcting plate 44 g and theradially outer portion thereof. Accordingly, the focal point to beformed by the laser beam passing through the radially inner portion ofthe condenser 44 a′ is displaced toward the chuck table 54 from thefocal point to be formed by the laser beam passing through the radiallyouter portion of the condenser 44 a′, wherein the laser beam is appliedto the wafer 10 in the condition where the focal point of the laser beamis set inside the wafer 10 to thereby form a shield tunnel. As a result,an effect similar to that in the above preferred embodiment can beachieved. That is, the energy of the laser beam can be effectively usedfor the formation of the shield tunnel, so that a plurality of goodshield tunnels can be continuously formed along each division line overthe entire length thereof.

The term of “the function of spherical aberration” does not mean thefunction obtained in only the case that a completely spherical convexlens is used to provide aberration. Essentially, this term includes thefunction of providing the configuration that the focal point to beformed by the laser beam passing through the radially inner portion ofthe condenser is displaced toward the chuck table from the focal pointto be formed by the laser beam passing through the radially outerportion of the condenser. Further, while the laser beam is applied tothe front side of the wafer where the SAW devices are formed in theabove preferred embodiment, the present invention is not limited to thisconfiguration. The laser beam may be applied to the back side of thewafer in the condition where the front side of the wafer is orienteddownward.

The present invention is not limited to the details of the abovedescribed preferred embodiment. The scope of the invention is defined bythe appended claims and all changes and modifications as fall within theequivalence of the scope of the claims are therefore to be embraced bythe invention.

What is claimed is:
 1. A laser processing apparatus comprising: a chucktable for holding a wafer; laser beam applying means for applying alaser beam to said wafer held on said chuck table; and a feedingmechanism for relatively feeding said chuck table and said laser beamapplying means; said laser beam applying means including a laseroscillator for oscillating said laser beam, and a condenser having afocusing lens for focusing said laser beam oscillated by said laseroscillator and applying said laser beam focused to said wafer held onsaid chuck table; said condenser having a lens configured to realizespherical aberration such that a focal point to be formed by said laserbeam passing through a radially inner portion of said condenser iscontinuously changed in position toward said chuck table from a focalpoint to be formed by said laser beam passing through a radially outerportion of said condenser, wherein said spherical aberration is realizedby said focusing lens having spherical aberration; said laser beam beingapplied to said wafer in the condition where the focal point of saidlaser beam is set inside said wafer so as to be continuously changed inposition along the thickness of said wafer to account for changes in thethickness of said wafer, thereby forming a shield tunnel inside saidwafer, said shield tunnel being composed of a fine hole and an amorphousregion surrounding said fine hole.
 2. The laser processing apparatusaccording to claim 1, wherein the range of change in position from thefocal point to be formed by said laser beam passing through the radiallyouter portion of said condenser to the focal point to be formed by saidlaser beam passing through the radially inner portion of said condenseris set to 50 to 2000 μm.
 3. A method of applying a laser beam to a waferhaving a plurality of division lines for dividing the wafer into aplurality of device chips, the method comprising: holding the wafer by achuck table; aligning at least one division line of the plurality ofdivision lines of the wafer with a laser beam applying means; setting afocal point of a laser beam emitted by said laser beam applying meansusing a focusing lens configured to realize spherical aberration,wherein the focal point to be formed by said laser beam passing througha radially inner portion of said focusing lens continuously changes inposition toward said chuck table from said focal point to be formed bysaid laser beam passing through a radially outer portion of saidfocusing lens; and applying said laser beam to said at least onedivision line using said laser beam applying means, wherein said focalpoint of said laser beam is set inside said wafer so as to becontinuously changed in position along a thickness of said wafer toaccount for changes in said thickness of said wafer, thereby forming aplurality of shield tunnels inside said wafer along said at least onedivision line, each of said shield tunnels including a fine hole and anamorphous region surrounding said fine hole.
 4. The method according toclaim 3, wherein the range of change in position of said focal point ofsaid laser beam toward said wafer from said focal point is set to 50 to2000 μm.
 5. The method according to claim 3, further comprising acondenser including said focusing lens, wherein said condenser isconfigured to produce said spherical aberration.
 6. The method accordingto claim 3, wherein the range of change in position from the focal pointto be formed by said laser beam passing through a radially outer portionof said condenser to the focal point to be formed by said laser beampassing through a radially inner portion of said condenser is set to 50to 2000 μm.
 7. The method according to claim 3, further comprising afocal point correcting plate located between said focusing lens and saidwafer for correcting the position of the focal point of said laser beamto be focused by said focusing lens.
 8. The method according to claim 7,wherein said focal point correcting plate is made of glass.
 9. Themethod according to claim 7, wherein said focal point correcting plateincludes a spherical convex surface.